Internal-combustion engines equipped with injection systems are known in motor vehicle technology. Self-ignitable internal combustion engines, in particular diesel engines, are also internal combustion engines with fuel injection systems. The operational performance of these engines is determined decisively by the injection process which may include, for example, injection quantity, time of injection, etc.
In order to exceed the nozzle opening pressure of an injection valve, it is known to pressurize the fuel that is to be injected into an internal combustion engine. This considerable pressure may be achieved using a high-pressure pump. When the fuel is pressurized, the nozzle needle of the injection valve opens, and the injection process begins.
To build up pressure, the high-pressure chamber of the high-pressure pump is sealed by a solenoid valve leading to a pressure-free chamber. If the injection process is to be ended before the pressure of the pressurized fuel falls below the closing pressure of the injection valve, the solenoid valve is opened which allows the pressure in the injection system to be reduced. The reduced pressure will cause the injection valve to close and the injection process to end.
It is known also, not to keep the solenoid valve constantly closed during the injection process, but rather to open it at least once briefly to a certain degree during the process and then reclose it. By opening and closing the solenoid valve in this manner, the fuel injection rate to influence the running of the engine may be adjusted.
In order to determine a defined beginning and end of injection process, a solenoid valve has very short switching times. An extremely effective control of the injection process results from control of the injection characteristic (opening and closing of the solenoid valve during the injection process), so that turn-on and turn-off time variances, which are dependent on the solenoid valve, have a drastic effect on the accuracy of metering the fuel.
Published German Patent Application 34 42 764 discloses a device for switching electromagnetic devices quickly. The apparatus described in this application has an electromagnetic device connected in series with a break device. A controllable capacitor is arranged parallel to the break device. The break device can be used for metering fuel in internal-combustion engines. The short switching times do in fact lead to a low power loss for the circuit arrangement, however, it may result in certain disadvantages. The apparatus discussed in this German application allows the load current to be operated in a fixed cycle. By switching the break device on and off, the load current is able to be lowered to a specific average value.